Apparatus and method for maintaining an operating condition for a blower

ABSTRACT

An apparatus and method for maintaining a predetermined flow rate in a ventilation system having a motor driven blower. In one embodiment, the blower is a draft inducer. The apparatus contains a module for determining the impedance at the outlet of the blower. The impedance results from the length of pipe through which the blower blows the exhaust from a heating device, such as a hot water heater along with any obstructions. The impedance is determined by using a look-up table to compare the measured rotation rate (RPMs) of the motor shaft to values in the look-up table. The RPMs are measured by a sensor that is coupled to the rotating shaft of the motor. The sensor provides a sensor signal to the impedance module. Once a substantially matching value is found, the corresponding impedance is obtained from the look-up table. The impedance is then passed to a module for setting the motor speed. The motor speed module adjusts the speed of the motor based on the impedance to maintain a constant flow rate.

PRIORITY

The present application claims priority from U.S. patent application Ser. No. 10/847,207 entitled “Draft Inducer Having a Backward Curved Impeller” that was filed on May 17, 2004 and also from U.S. provisional application 60/499,844 that was filed on Sep. 3, 2003. The present application incorporates both applications herein by reference in their entirety.

RELATED APPLICATIONS

The present application is related to co-pending U.S. application Ser. No. ______ filed on even date therewith, entitled “Draft Inducer Performance Control” and naming Fred A. Brown and Gregory R. Turi as the inventors, the disclosure of which is incorporated herein, in its entirety, by reference.

TECHNICAL FIELD AND BACKGROUND ART

The present invention relates to devices that move air by spinning such as blowers and/or fans including draft inducers. Such device may be used with HVAC systems, furnaces and hot water heaters for venting air or gases from an inlet to an outlet.

Prior art ventilation systems for HVAC (heating, ventilation, and air conditioning), furnaces and hot water heaters are designed to extract a requisite amount of gas from the system regardless of the output impedance. Stated in another way, a ventilation system for a hot water heater that includes a draft inducer is designed such that the system will operate assuming a maximum impedance at the output of the draft inducer. As such, if the impedance at the output of the draft inducer is less than the maximum, heat that could be used for heating is removed from the system making the system less than ideal in terms of energy efficiency and cost. If too little heat is removed the system will operate inefficiently and create possible negative emissions. Negative emission may be levels of exhaust gas above that specified by a manufacturer or by a government body. If the flow rate is not high enough, the exhaust may stagnate and extinguish the heating source. AC motors and forward curved impeller blades were used by prior art systems in draft inducers for hot water heaters because AC motors exhibit a flat torque/speed curve. As such, even if the torque drops, the speed of the motor remains nearly constant. This characteristic was viewed as desirable, since the impedance on the draft inducer from application to application varies. For example, the length of the exhaust pipe that the draft inducer is used to drive may vary from approximately 1 foot to lengths of 45 feet or more. As such, the draft inducer with an AC motor would be able to drive the gases through the exhaust piping regardless of the length of the exhaust pipe.

SUMMARY OF THE INVENTION

An apparatus for maintaining a predetermined flow rate in a ventilation system having a motor driven blower is disclosed. In one embodiment, the blower is a draft inducer. The apparatus contains a module for determining the impedance at the outlet of the blower. The impedance results from the length of pipe through which the blower blows the exhaust from a heating device, such as a hot water heater along with any obstructions. The impedance is determined by using a look-up table to compare the measured rotation rate (RPMs) of the motor shaft to values in the look-up table. The RPMs are measured by a sensor that is coupled to the rotating shaft of the motor. The sensor provides a sensor signal to the impedance module. Once a substantially matching value is found, the corresponding impedance is obtained from the look-up table. The impedance is then passed to a module for setting the motor speed. The motor speed module and the impedance module may be part of a single processor. In other embodiments the motor speed module and the impedance module may be part separate processors. In other embodiments the motor speed module and the impedance module may be firm ware that is partially hardware and partially software.

The motor speed module adjusts the speed of the motor based on the impedance to maintain a constant flow rate. A look-up table is accessed which contains RPM values for given impedances that will maintain the preferred flow rate. In one embodiment the preferred flow rate is between 26 and 27 cubic feet per minute (cfm). Based on the difference between the measured RPM value and that found in the look-up table the motor speed is adjusted until the two RPM values are equal. The motor speed can be adjusted by changing the pulse-width modulated signal that is sent from the processor to the motor.

In other embodiments, the blower is part of a heating system, such as an HVAC (heating, ventilation, and air conditioning) system. When the impedance is determined, the module for determining the impedance, may output a signal that indicates the length of pipe that is coupled to the outlet of the blower.

The apparatus performs the following methodology in order to adjust the flow rate to a desired flow rate after the blower has started. First, the rate of rotation of the motor is measured. The measured rate of rotation of the motor is compared to an empirically measured rate that is in a look-up table. The rate of rotation is then adjusted until the measured rate of rotation matches the empirically determined rate from the look-up table. The rate of rotation of the motor is adjusted by changing the duty cycle of a pulse width modulated signal that is provided to power the motor. The look-up table that is used has an association between empirically determined rotation rates and impedance attached to the outlet of the blower. In one embodiment, the flow rate is adjusted such that the flow rate produces a substantially optimal energy efficiency for the heating device that is coupled to the inlet of the blower. The flow rate allows impurities to be removed from the hot water heater without having the exhaust become stagnant, while not drawing out an excessive amount of heat from the heating device. Thus, the system is energy efficient.

In certain embodiments of the invention, prior to adjusting the rotational rate to obtain a desired flow rate, the system determines the impedance that is attached to the system. For example, the impedance may be the length of pipe that is attached to a draft inducer and may include any other impedance, such as any blockage that is within the pipe. The system can automatically determine the impedance that is attached based upon previously determined empirical information. Without knowing the length of pipe/impedance that is coupled to the system, a processor will provide a power signal to a motor. The power signal may be in the form of a PWM signal. The processor receives a sensed rotational signal from the sensor attached to the motor that measures the rotational rate of the shaft of the motor. The rotational rate is then compared to a look-up table that associates rotational rates with impedances for the power signal. Based on the measured rotational rate, the impedance is determined. In certain embodiments if the rotational rate is between a first and a second value that are found in the look-up table, the processor will select the rotational rate associated with the larger impedance. This is done so as to provide a slightly larger flow rate than the ideal flow rate rather than a flow rate that is less than the ideal, thus the system at least meets any government flow requirements or heating manufacturer's suggested flow requirements. In preferred embodiments the motor is a DC motor.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing features of the invention will be more readily understood by reference to the following detailed description, taken with reference to the accompanying drawings, in which:

FIG. 1 is schematic drawing of a ventilation system using a blower;

FIG. 2 is a schematic diagram showing a draft inducer.

FIG. 3 is a block diagram showing an impedance module and a motor speed module;

FIG. 4 is a graph showing torque speed curves and impedance curves for a motor;

FIG. 5 is an exemplary look-up table showing the values in the look-up table for a 60% duty cycle;

FIG. 6 is a flow chart showing the steps that are taken by the module impedance module; and

FIG. 7 is a flow chart showing the steps that are taken by the module for setting the motor speed.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Definitions. As used in this description and the accompanying claims, the following terms shall have the meanings indicated, unless the context otherwise requires: the term ‘backward curved blade’ is a term of art as understood by those in the art of impeller construction. As used herein, the term ‘blower’ shall imply all devices that move air by spinning including fans.

In one embodiment, the invention automatically determines an impedance that is coupled to the output of a blower that is part of a ventilation system. After determining the impedance, the system uses the impedance to determine the duty cycle for a motor that is powered using pulse-width-modulated (PWM) waveforms, such that the flow rate through the ventilation system to the outside environment will be nearly optimal in terms of energy conservation given that the ventilation system must exhaust a minimal amount of fluid. The minimal amount of fluid that the ventilation system must exhaust is determined either by statute or a manufacturer's specification for a heating element, such as a hot water heater, furnace, or heating component of a HVAC system.

FIG. 1 is schematic drawing of a ventilation system 10 using a blower. In this example, the ventilation system is coupled to a hot water heater 20 for collecting the output of the hot water heater and moving the output 25 from an inside environment 30 to an outside environment 40. It should be recognized that a hot water heater is being employed as an exemplary embodiment of a heat source and that the invention may be employed with other systems that need to vent an exhaust of a heating or cooling source to an outside environment. As shown, the inside environment 30 could be a structure such as a building and the outside environment 40 may be the atmosphere. The inside environment 30 is therefore a space that is volumetrically smaller than the outside environment 40. The outlet of the hot water heater is coupled to the inlet of a draft inducer 50. The draft inducer 50 includes a motor with an attached blower within a housing. An example of a draft inducer is shown in U.S. patent application Ser. No. 10/847,207 entitled “Draft Inducer Having a Backward Curved Impeller” that was filed on May 17, 2004 and that is incorporated herein by reference in its entirety. The draft inducer 50 draws air in from the inside environment 30 into a mixing chamber where it mixes with the exhaust output of the hot water heater. During mixing, the air from the inside environment 30 cools the exhaust output. The draft inducer 50 also causes the mixed fluid to be directed toward the outside environment through outlet piping 60. The voltage supply that is provided to the motor of the draft inducer is preferably regulated. By regulating the voltage supply, the motor performance is immune to input voltage variations. By maintaining a constant DC voltage to the motor, the proportionality between the duty cycle of the PWM waveform that is provided to the motor and the resulting RPM of the motor are very accurately related.

If the draft inducer draws in too much of the exhaust into the mixing chamber, energy is removed from the hot water heater that could be used for heating the water. If too little exhaust is removed from the hot water heater, the exhaust will stagnate and potentially extinguish the heating source. In order to balance the amount of heat that must be removed from the hot water heater as prescribed by law or as designated by the manufacturer of the hot water heater and to increase energy efficiency, the motor of the draft inducer is controlled by a processor.

The impedance that is coupled to a draft inducer at its outlet varies from application to application. The draft inducer may be installed in a system having exhaust piping 5 feet in length and in other embodiments the exhaust piping may be 50 feet in length. In order to properly regulate the flow rate through the ventilation system so as to increase the energy efficiency over prior art system, the impedance is determined by the processor. The processor performs a routine in combination with a sensor that is attached to the motor of the draft inducer to automatically determine the impedance of the system.

FIG. 2 is a schematic diagram showing a draft inducer 200. The draft inducer 200 includes an impeller 210 that spins about the shaft 215 of a motor 220. The motor 220 is powered by the processor 235. In a preferred embodiment a DC motor is used with the draft inducer because a DC motor exhibits a steep torque/speed curve and therefore variations in speed can be equated to changes in torque. In such an embodiment, the processor is coupled to the motor by way of two connections. The processor receives a sensor signal through a sensor lead 225 from a sensor that is attached to the motor 220. This sensor signal provides information regarding the rotational speed of the shaft of the motor 220. The processor uses this information to determine the duty cycle of the pulse width modulated signal that is to be employed in powering the motor through the power lead 230. As shown in FIG. 2, the exhaust 240 of the heating device enters the draft inducer the inlet 245 of the draft inducer 200. The impeller blades 250 of the draft inducer spin drawing in the exhaust 240 and mix the exhaust with ambient air 255, thereby cooling the exhaust. The impeller blades 250 direct the mixture to an outlet of the draft inducer 260 that is coupled to an exhaust pipe that leads to the outside environment. The spinning of the blades of the impeller creates a flow of fluid to the outside environment and therefore the flow rate is controlled by the power that is supplied to the motor.

The processor 235 as shown in FIG. 3 uses two modules that may be either circuitry, software code or a combination of the two (firmware). If the modules are hardware based, the modules could be part of the processor. The processor using the impedance module 310 first determines the impedance that results from the length of pipe that is attached to the draft inducer along with any obstructions within the length of pipe. The impedance is automatically determined preferably prior to the heating source being turned on.

The impedance module 310 causes the processor 235 to set the duty cycle of the PWM waveform for the motor. This duty cycle could be any value, however it is preferable to begin the PWM waveform at approximately a 50% duty cycle in order to create enough air flow through the ventilation system prior to turning on the heating source to avoid any problems with the energy source due to back ignition. The sensor signal produced by the rotation of the shaft of the motor is received by the processor 235 and this measured rotational speed is compared to values in a look-up table 330 that are stored in memory 340 that is associated with the processor 235. The values in the look-up table 330 are empirically determined values and are the intersection points of the torque-speed curves for a DC motor and the impedance curves. The look-up table can include three associated values: 1. the duty cycle (i.e. 60%, 70% etc.), 2. the RPM value (i.e. 2000 RPM, 3000 RPM etc.) and 3. the impedance (i.e. the equivalent length of exhaust pipe 10 Ft., 20 Ft. etc.). Thus, by knowing the duty cycle and measuring the RPM value the length of pipe that is attached to the draft inducer can be determined, which is the equivalent impedance.

FIG. 4 shows a series of torque speed curves and impedance curves for a motor and exhaust piping having a known diameter. For each diameter of exhaust piping, a different set of curves would exist. The torque speed curves are relative to the duty cycle of the PWM waveform and are shown with respect to 10% increments from 0 to 100% duty cycle. These curves are provided to show how the three data values are determined in the look-up tables. The units of the x-axis are the percentage of the duty cycle of the PWM waveform and the units of the y-axis are RPMs. As shown in FIG. 4, for a duty cycle of 50% and a measured RPM value of 2000 RPM, the impedance is equivalent to 15 feet of pipe.

FIG. 5 shows an exemplary look-up table showing the values in the look-up table for a 60% duty cycle. The data within the table is empirically determined by attaching a known length of exhaust pipe to the draft inducer and then cycling through all of the PWM values and measuring the RPM level. The table can then be used with a similar draft inducer having the same diameter exhaust pipe to determine the length of pipe that is attached to the draft inducer. The unknown length of exhaust pipe is determined by measuring the RPM value and correlating that with the duty cycle and the impedance. If the measured RPM value is between two RPM values in the table, the system can interpolate between the two to determine the impedance. For example, if the measured RPM value was 1750 RPM then the system would interpolate between the impedances associated with 1500 RPMs and 2000 RPMs to determine that there is 12.5 feet of pipe that is causing an impedance at the output to the draft inducer. In other embodiments, if the RPM value is not exactly the same as the empirically determined intersection point, the processor could select the larger of the two impedances that are closest to the measured RPM value. In the provided example, the module would select the 2000 RPM group and the associated 15 feet of pipe. The larger impedance provides a margin of error when the processor finally sets the PWM signal for the motor. Thus, by selecting the higher impedance the flow rate is guaranteed to be above the minimum that is required either by the manufacturer of the heating/cooling device or by law.

The second module within the processor is the module for setting the motor speed (the motor speed module) 320. This module 320 accesses the memory associated with the processor and finds a look-up table 330 that contains a listing of PWM settings that are each associated with a different impedance for the empirically determined near optimal flow rate. This look-up table 330 can be part of the look-up table that is employed by the impedance module or may be a separate look-up table. In one embodiment, the optimal flow rate is approximately 26-27 CFM for a 4 inch diameter outlet pipe. Based upon the determined impedance the PWM signal is selected and the processor provides the PWM signal to the motor. For the given PWM signal, the motor speed module expects that the sensor will sense a signal that is equivalent to an expected RPM value. If the measured RPM value is not equivalent to the expected RPM value the module will adjust the duty cycle of the PWM waveform. If the RPM value is less than expected RPM value, the duty cycle will be increased. If the RPM value is greater than the expected RPM value, the duty cycle will be decreased. The motor speed module continues this process until the measured RPM value equals the expected RPM value. Even after the measured RPM value and the expected RPM value are equal, the system will continue to measure and adjust for any fluctuations that occur.

FIG. 6 is a flow chart showing the steps that are taken by the module impedance module. First the module sets the duty cycle of the PWM signal (600). Preferably, the duty cycle is set between 40% and 60% i.e. 50%. The module then receives the sensor signal from the sensor that is attached to the motor (610). The sensor may be built into the motor and may be part of the stator or rotor. The sensor senses the revolutions of the shaft of the motor. The number of revolutions per minute are determined within the module based on the signal (620). The revolutions per minute and the duty cycle are then used to determine the impedance based upon a look-up table that contains the intersection points of FIG. 4 (630). If the RPM value falls between empirically measured RPM values within the look-up table and therefore between two impedance levels, the greater impedance level is selected. It should be understood by one of ordinary skill in the art that rotational measurement signal may be converted into any format that provides information about the rotational speed and that revolutions per minute are used in this application for convenience.

FIG. 7 is a flow chart showing the steps that are taken by the module for setting the motor speed. After the impedance is determined by the impedance module, the impedance is received by the motor speed module (700). The motor speed module accesses a look-up table in memory and locates the duty cycle for the impedance that will produce a near optimal flow rate that removes a minimal amount of energy from the heating element (710). As previously stated the heating element may be a hot water heater, a furnace or a heating element in an HVAC system for example. Also associated with the duty cycle is the expected RPM rate of the shaft. The processor provides the appropriate duty cycle to the motor and the sensor senses the revolutions of the shaft. The sensor signal is provided to the motor speed module (720). The motor speed module compares the RPM rate of the shaft to the expected RPM rate. The motor speed module inquires if the measured RPM rate is equal to the expected RPM rate (730). If it is the process ends. In certain embodiments, the process can continue to check and compensate for any deviations in RPM. The motor speed module then checks to see if the RPM rate is lower than the expected RPM rate (732). If it is low, the motor speed module increases the duty cycle (733). If the measured RPM rate is not lower than the expected RPM rate, then the motor speed module decreases the duty cycle (735). As previously stated, the measured RPMs will be provided to the motor speed module on an ongoing or periodic basis to compensate for any fluctuations that may occur during the operation of the blower and the heating element in the ventilation system. It should be understood by one of ordinary skill in the art that the steps of the previous flow charts may be implemented in a different order without deviating from the purpose of the invention.

Additionally, the processor can include an alarm function in the event that the measured RPM value exceeds a predetermined upper threshold or is less than a separate predetermined lower threshold. If either condition occurs, the processor will set off an alarm that may be audible and/or visual. The alarm will also shut down the hot water heater or other heating device and turn off the motor associated with the draft inducer. As the RPM level passes the upper threshold level the impedance is greater than expected and this indicates that a blockage exists in the exhaust piping. Similarly, if the RPM value is too low and goes below the lower threshold, the system assumes that either there is a hole in the exhaust pipe or the exhaust pipe has been removed, since the impedance is much lower than expected and again the processor will shut down the heating element and will turn off the motor.

In an alternative embodiment, the invention may be implemented as a computer program product for use with a computer system. Such implementation may include a series of computer instructions fixed either on a tangible medium, such as a computer readable media (e.g., a diskette, CD-ROM, ROM, or fixed disk), or transmittable to a computer system via a modem or other interface device, such as a communications adapter connected to a network over a medium. The medium may be either a tangible medium (e.g., optical or analog communications lines) or a medium implemented with wireless techniques (e.g., microwave, infrared or other transmission techniques). The series of computer instructions embodies all or part of the functionality previously described herein with respect to the system. Those skilled in the art should appreciate that such computer instructions can be written in a number of programming languages for use with many computer architectures or operating systems. Furthermore, such instructions may be stored in any memory device, such as semiconductor, magnetic, optical or other memory devices, and may be transmitted using any communications technology, such as optical, infrared, microwave, or other transmission technologies. It is expected that such a computer program product may be distributed as a removable media with accompanying printed or electronic documentation (e.g., shrink wrapped software), preloaded with a computer system (e.g., on system ROM or fixed disk), or distributed from a server or electronic bulletin board over the network (e.g., the Internet or World Wide Web).

Although various exemplary embodiments of the invention have been disclosed, it should be apparent to those skilled in the art that various changes and modifications can be made which will achieve some of the advantages of the invention without departing from the true scope of the invention. These and other obvious modifications are intended to be covered by the appended claims. 

1. An apparatus for maintaining a predetermined flow rate in a ventilation system having a motor driven blower, the apparatus comprising: a module for determining an impedance at the output of the blower; a module for setting the motor to a speed that will maintain the predetermined flow rate, wherein the speed is determined based upon the impedance.
 2. The apparatus according to claim 1 wherein the blower is part of a draft inducer.
 3. The apparatus according to claim 2 wherein the input of the blower is coupled to an outlet of a hot water heater.
 4. The apparatus according to claim 1 wherein in the module for determining, the module determines an impedance that is equivalent to a length of pipe attached to the output of the blower.
 5. The apparatus according to claim 1 wherein the module for determining and the module for setting both use a processor
 6. The apparatus according to claim 5 wherein the processor is shared
 7. The apparatus according to claim 1 wherein the apparatus includes a sensor for sensing rotation of a shaft of the motor
 8. The apparatus according to claim 1 wherein the module for setting the motor accesses a look-up table having a set of values to determine a speed for the motor based upon the impedance.
 9. A method for maintaining a flow rate at an outlet to a blower having a motor, the method comprising: measuring the rate of rotation of the motor; and adjusting the motor until the measured rate of rotation substantially matches an empirically determined rate.
 10. The method according to claim 9, wherein adjusting the motor includes changing a duty cycle of a pulse width modulated waveform.
 11. The method according to claim 9, further comprising determining an impedance coupled to the outlet of the blower by comparing the measured rotation rate to empirically determined rates in a look-up table wherein the look-up table has an association between an empirically determined rate and an impedance.
 12. The method according to claim 9 further comprising: selecting the empirical rate wherein the empirical rate produces a flow rate producing substantially optimal energy efficiency of a hot water heater, wherein the hot water heater is coupled to the inlet of the blower.
 13. The method according to claim 12, further comprising determining an impedance coupled to the outlet of the blower using a look-up table that associates each empirically determined rotational rate with an impedance by comparing the measured rate of rotation to the empirically determined rates within the look-up table.
 14. The method according to claim 11 wherein the rate of rotation of the motor produces a flow rate that removes a substantially minimal amount of heat from a heating device coupled to the blower.
 15. The method according to claim 14, wherein the heating device is a hot water heater.
 16. A method for producing a near optimal energy efficient flow rate using a blower powered by a motor between a heat producing source and an outside environment, the method comprising: setting a duty cycle for a DC motor to a preset level using a processor; sampling the revolution rate of the DC motor; comparing the sampled revolution rate to a value in a look-up table; if the sampled revolution rate is not within a predetermined range of the value in the look-up table, varying the duty cycle of the DC motor until the sampled revolution rate matches the value in the look-up table.
 17. The method according to claim 16, further comprising: determining an impedance at the outlet of the blower.
 18. The method according to claim 17, wherein determining an impedance includes obtaining a look-up table from memory containing associations between duty cycle, revolution rate and impedance; and selecting an entry within the look-up table having the closest revolution rate to the sampled revolution rate.
 19. The method according to claim 16, wherein the predetermined range is zero and therefore requires an exact match.
 20. The method according to claim 16, further comprising: providing to a processor a diameter of an exhaust pipe to be used with the blower.
 21. The method according to claim 16 wherein the blower is a draft inducer.
 22. The method according to claim 21, wherein the draft inducer is coupled on a first side to a hot water heater and on a second side to the impedance.
 23. The method according to claim 22 wherein the impedance is a length of pipe.
 24. A system producing a near optimal energy efficient flow rate at an outlet of a blower, the system comprising: a processor producing a pulse width modulated signal having a duty cycle; a DC motor having a shaft and receiving the pulse width modulated signal causing the shaft to rotate; a blower coupled to the DC motor a rotation sensor sensing the rotations and outputting a signal representative of the rotations; wherein the processor receives the output signal from the rotation sensor, determines the number of revolutions per a time period that the shaft rotates, and adjusts the pulse width modulated signal until the number of revolutions per time period reaches an empirically determined value.
 25. The system according to claim 24, wherein the empirically determined value is associated with an impedance.
 26. The system according to claim 25, wherein the processor determines the impedance at the outlet by determining the revolutions per time period and compares the determined revolutions to a look-up table containing an association between revolutions per time period and impedance.
 27. The system according to claim 26 wherein the processor accesses a look-up table and based upon the impedance determines the empirically determined value. 